FIG. 1 is a schematic view of a conventional seven-cavity klystron 10. The klystron 10 amplifies RF energy by extracting energy from an electron beam. A plurality of successive drift tubes 30, 31, 32, 33, 34, 35 respectively connect with the seven cavities 16, 17, 18, 19, 20, 21, 22, such that one drift tube interconnects two adjacent cavities. Each cavity is individually tuned, and an electromagnet is placed around the klystron for focusing the electron beam. The electron gun 12 produces an electron beam 11 which is accelerated to a high voltage. Simultaneously, a microwave signal is fed into an RF input port 14 for interacting with the electron beam 11 within an input resonating cavity 16. The electron beam 11, with velocity modulation superposed by the input microwave signal, passes through a sequence of successive gain cavities 17, 18, 19, 20, 21. The velocity modulation of the electron beam is amplified as it passes by each of the gain cavities 17, 18, 19, 20, 21. The velocity modulated electron beam travels to the output cavity 22, where the velocity modulation is converted into amplified microwave output power and is extracted through the RF output port 24. The spent electron beam is absorbed by the collector 26 positioned after the output cavity.
The velocity modulation is also known as bunching which is caused by the oscillating electric fields applied to the drift tubes. As each sequential cavity is encountered, electrons are accelerated during opposing electric fields and pass through the drift tubes 30, 31, 32, 33, 34, 35 and other electrons are slowed during a corresponding electric field. This cycling causes the electrons to be grouped into bunches at the input frequency. The geometries of the cavities 16, 17, 18, 19, 20, 21, 22 along the length of the klystron are designed to enhance the bunching of electrons. The spacing between successive cavities 16, 17, 18, 19, 20, 21, 22 is also intended to optimize the electron bunching and improve the output power of the klystron 10.
Solid electron beam klystrons require a high beam voltage to keep the interaction efficiency high. The circuit length of the klystron is proportional to the electron beam voltage. Thus, a high beam voltage requires a larger klystron having a longer circuit length to keep the interaction efficiency. Another problem with high beam voltage is the increased probability of RF arcing which can damage or destroy a klystron. What is needed is an improved klystron that addresses operation at a lower beam voltage to reduce the required circuit length and reduce the probability of RF arcing.